The present invention relates to gussetted plastic bags which incorporate relief seals extending out of the existing seal positioned at the gusset joint and to a method for forming such bags. More particularly, this invention relates to a pair of relief seals formed adjacent each gusset joint between the corresponding gusset wall and lower sidewall, each relief seal is positioned below a relief line extending from the gusset joint at an angle of about 45.degree. from the lower sidewall.
Disposable or reusable and reclosable plastic bags capable of being compactly stored as a food container in freezers, yet capable of standing with an upright spout in a microwave oven for non-spill thawing and cooking of food contained in the bag are known. In that regard, reference is made to Stevenson (U.S. Pat. No. 4,358,466), assigned to the same assignee as the present invention. Stevenson discloses a bag formed of two wing-shaped pouches on either side of an upright spout. The bag preferably has inner and outer skin layers of different heat-softening temperature responses so that peripheral sealing is effected only with the inner layers, allowing the wing-shaped pouches to be formed out of a gussetted bottom without sealing of the outer layer plies facing one another. While the Stevenson bag has a unique gussetted bag structure it has other aspects found in the typical gussetted plastic bag.
Typically, gussetted plastic bags have had problems with strength and durability at the gusset region. Seepage and pinholing are the usual results when the gussetted plastic bag is filled with product. Consequently, there has been a need to reinforce the gusset region of gussetted plastic bags. Furthermore, gussetted plastic bags have lacked the ability to stand upright when empty and when being filled with product. Consequently, there has been a need to strengthen the gusset portions of the bag to render them self-standing. Several approaches to these problems have been taken previously.
For example, Bustin (U.S. Pat. No. 4,353,497) and Sanicki (U.S. Pat. No. 4,658,433) disclose using heat seals to add strength to the bag. However, these seals waste a large portion of bag volume, and therefore reduce storage capacity. Similarly, Takahashi et al (U.S. Pat. No. 3,483,061) disclose a bag with a convex arcuate bottom for the purpose of bagging uniquely shaped products without damage to the bag. Likewise, Ericson (U.S. Pat. No. 3,508,473) describes a bottom seal which has two adjacent angular seals on either side joined by an arcuate seal into a continuous seal from one edge to the other. But, the Ericson and Takahashi et al patents both possess potential seepage and pinholing problems at the bottom corners.
As mentioned, stand-up plastic bags also often involve various forms of bag reinforcement. See for example Plate (U.S. Pat. No. 4,046, 065), Kan (U.S. Pat. Nos. 3,980,225 and 4,055,109), Kugler (U.S. Pat. No. 3,395,622) and Arai (U.S. Pat. No. 3,839,128) all of which represent typical prior art relating to self-standing plastic bags. More specifically, they show attempts to reinforce gussetted regions via diagonal heat seals extending from the weak points down to the bottom of the bag. Similarly, the Kirkpatrick (G.B. Patent No. 2,047,199) patent discloses a spot heat seal which joins together all four layers of the gusset fold and wall portions. Finally, reference is made to copending applicaton Ser. No. 171,053, filed Mar. 21, 1988, assigned to the same assignee as the present invention, which shows an improved reinforced stand-up plastic bag structure. While such stand-up plastic bags may have reinforcements in the gusset region, such reinforcements are not necessarily designed to provide additional strength and durability, but rather are generally designed to provide a more stable stand-up base.
It is perceived that additional improvements are needed in reinforcing gussetted plastic bags before consumers will have a gussetted plastic bag which withstands normal functional use. Thus, there is a need to increase bag strength and durability, especially around the gusset region, in order to prevent pinholing and seepage when the bag contains product. There is also a need for an efficient and less expensive method of making a gussetted plastic bag.